Dictionary Attack

A dictionary attack is form of lookup attack used to crack passwords even when the passwords have been obscured with a hash function. There are legitimate uses for a dictionary attack, like recovering forgotten passwords or verifying that passwords are secure, but dictionary attacks are usually used for nefarious purposes, and understanding the process is necessary for protecting yourself against it.

Process

This section describes the steps necessary to perform a dictionary attack.

Step 1: Steal a Password Table

This is the most difficult part of the attack, as system administrators don't just give away user's passwords. Obtaining a password table means being able to defeat a system's security or buying a password table from someone who has. Since it is a security flaw for even admins to know a user's password, the passwords in the table are almost always obfuscated through a one-way hash function. So, when you look at a password table, it may look something like this:

User

Password

smithr

5EBE2294ECD0E0F08EAB7690D2A6EE69

jonesd

0CC175B9C0F1B6A831C399E269772661

doej

92EB5FFEE6AE2FEC3AD71C777531578F

johnsonk

25D55AD283AA400AF464C76D713C07AD

williamsd

21232F297A57A5A743894A0E4A801FC3

In this case, the passwords have been run through an MD5 hash function which cannot be reversed. When a user logs in, the computer will first apply the hash to the password they typed in, and compare it with the one on-file to ensure a match. This way, the plaintext password is only ever known by the user. So then, how can a password cracker ever determine a user's password without a Brute Force Attack? This is where the dictionary attack comes into play.

Step 2: Obtain a List of Commonly Used Passwords

Step 3: Run the Same Hash On the List of Common Passwords

Hash functions used to obfuscate passwords cannot be reversed, but they can be repeated. To do this, a cracker must know the exact same hash function used to obfuscate the passwords in the stolen password table. This isn't too difficult, since only a few are commonly used. From here, the cracker runs all the commonly used passwords through the same hash function and gets a list of the hashes for each of the passwords.

Common Password

MD5 Hash

12345678

25D55AD283AA400AF464C76D713C07AD

admin

21232F297A57A5A743894A0E4A801FC3

god

A4757D7419FF3B48E92E90596F0E7548

password

5F4DCC3B5AA765D61D8327DEB882CF99

secret

5EBE2294ECD0E0F08EAB7690D2A6EE69

I should point out that the MD5 hash function is not a cryptographically secure hash function and should never be used to store passwords. I'm just using it for this example.

Step 4: Compare Against the Password Table

Once the dictionary of hashes has been generated, the cracker need simply look for matches in the hashes between the stolen password table and their dictionary. A match indicates that the user chose the password in the table.

User

Password

Match From Dictionary

smithr

5EBE2294ECD0E0F08EAB7690D2A6EE69

secret

jonesd

0CC175B9C0F1B6A831C399E269772661

-no match-

doej

92EB5FFEE6AE2FEC3AD71C777531578F

-no match-

johnsonk

25D55AD283AA400AF464C76D713C07AD

12345678

williamsd

21232F297A57A5A743894A0E4A801FC3

admin

This method will identify every password in the stolen password table that matches one in the dictionary of commonly used passwords, which, in common practice, is often over half.

Since computers are very fast at generating hash functions and comparing values between two tables, a dictionary attack with millions of commonly used passwords can be carried out against a table of millions of user passwords in a matter of minutes.

Defenses

This section details the ways to protect yourself from a dictionary attack.

Eliminate Access

It should go without saying that a system administrator should eliminate access to the password table for all but the most trusted personnel. As a user, you should also be wary of supplying passwords to any system where administrators don't vigilantly protect the password table. This is also a general rule for all information, no matter how encrypted you might feel your data is, never allow people to have access to it. You never know if it will fall into the hands of a clever person.

Use Unique Passwords

Only use unique passwords that won't show up in the cracker's dictionary. If you're not sure if your password is unique, do a Google search of your password in quotes. If there are any results, you should probably choose a more complicated password. Also, adding a number to the end of a common word will not make it safe, even if there isn't a Google hit. Multiple dictionaries can be created with permutations of existing passwords. For example, a cracker could make a copy of their dictionary and append the number 1 to the end of each password and run a dictionary attack on that as well and it wouldn't add much time to the process.

Use Uncommon Characters

Older systems rarely allow for this, but modern systems do. When you create a password, include a character that is not found on your keyboard. You can do this when typing in your password by holding down the ALT key on your keyboard then typing the character's number on your keypad (the numbers on the side of your keyboard, not the top), then release the ALT key. For example, ALT+175 gives you this character, "»". Few password dictionaries are going to include even a single password with that character in it, so your password will be safe.

Salt

In order to combat dictionary attacks, most modern password tables first apply a salt to passwords before sending them through a hash function. A salt is a modification to a password so that it will yield a completely different hash from what is expected. For example, the passwords below have been salted by adding a question mark to the beginning of them before being hashed, which yields a completely different MD5 hash.

Common Password

MD5 Hash

Salted Hash

12345678

25D55AD283AA400AF464C76D713C07AD

9F7128DB15B794862C8E96A819379D94

admin

21232F297A57A5A743894A0E4A801FC3

C0D28ABB7D0C94329B81AA112518ADA0

god

A4757D7419FF3B48E92E90596F0E7548

27DE47E3C4C4205ED02216C2A51AF071

password

5F4DCC3B5AA765D61D8327DEB882CF99

1007980A168F839AA8C4689C7FBDDB0E

secret

5EBE2294ECD0E0F08EAB7690D2A6EE69

D3A4FE3D71CD8546AA9BEC89F0D686DF

When using a salt, the program that handles user passwords must not only apply the salt when storing the password into the password table, but it must also apply the salt to the password each time a user logs in to make sure it will match the hash in the password table.

A salted password table adds a layer of security because, in order for a dictionary attack to work, the dictionary of hashes must be re-generated with the passwords and the salt. So, if the cracker doesn't know the salt, all of the password will fail to match the hash. However, if a cracker defeated a system's security well enough to make a copy of the password table, they probably also made a copy of the program that adds the salt. Because of this, a salt will not stop a cracker, because they will be able to determine how the salt was added and apply it to their password dictionary, but it might slow them down long enough for the administrator to take precautionary actions.